The following is an example of a 90%+ furnace. Things may differ slightly from manufacturer to manufacturer, but they’ll all be pretty close, and theory will remain the same.

• Furnaces equipped with a control board are continually checking to confirm rollout switches and the high-limit switch are all closed. It also sends 80V – 110V to the flame sensor to prove there’s no flame present before ignition as well with no call for heat
• The thermostat has a call for heat and closes R+W switch
• The control board completes a safety check to ensure rollouts and high-limit are closed (completed circuit). If a high-limit or flame rollout is open, or a pressure switch is closed before running the combustion blower, the control board will display an error and run the blower to get the attention of the homeowner
• Combustion blower starts up to ensure exhaust venting, primary heat exchanger, secondary heat exchanger, combustion air, and drain lines are clear. The contacts of the pressure switch will close (to complete the safety circuit) if no blockages exist. If the combustion blower cannot close the contacts of the pressure switch, an error will be displayed. The combustion blower will try a couple more times before going into a lockout
• The igniter will start to warm up for around 10 – 20 seconds
• The trial for ignition begins. The gas needs to ignite the burners and flame sensor sense the flame. The gas valve will energize for about 7 seconds and ignite the burners
• The flame will be proven using flame rectification. If the flame is not proven within a few seconds, the gas valve de-energizes and the process starts again. The furnace usually tries three times before going into lockout and displaying an error
• Assuming flame is proven, the blower will begin after the heat exchanger has had time to warm up. Waiting for the heat exchanger to warm up helps not to blow cold air on the homeowner. There’s usually a delay of 30 seconds or so
• Once the thermostat is satisfied, the gas valve stops the flow of gas, and the blower will continue to run for about 90 seconds to cool off the heat exchanger and use the rest of the heat from the heat exchanger
• The combustion blower runs for a short time as well to endure the exhaust is cleared